The subject matter disclosed herein relates to turbomachinery systems. Specifically, the embodiments described herein relate to controlling the speed of a turbine in a turbomachinery system.
In a turbomachinery system, such as a gas turbine system, a starting motor and torque converter may be coupled, for example, in series to the turbine to accelerate the turbine. The starting motor provides an input torque to the torque converter, which converts the input torque to an output torque that is provided to the turbine and causes a turbine shaft to rotate. The torque converter contains guide vanes, with the position of the guide vanes determining the amount of torque that is converted and subsequently the speed of the turbine. A control system in the turbomachinery system may control the starting motor and torque converter to manage the turbine speed according to a desired acceleration profile or steady-state condition.
In current turbomachinery systems that employ a starting motor and torque converter, once the system has started operation and the torque converter has been filled with fluid, there may be a minimum threshold for the turbine speed. This minimum speed may be defined by the drive line mechanical limitations of the turbine and may be met when the guide vanes of the torque converter are closed to a minimum position (e.g., minimum setpoint). However, there may be situations in which it would be beneficial to reduce the speed of the turbine further below the minimum setpoint. For example, in emergency situations, it may be useful to reduce the turbine speed below the minimum instead of performing a complete shutdown of the turbomachinery system. In another example, reducing the turbine speed below the minimum setpoint may provide an opportunity to gather validation and testing data (e.g. stall validation and compressor mapping) that may be useful to operators and engineers in making informed decisions. Additionally, reducing the turbine speed below the minimum setpoint may extend the operating range of the turbomachinery system such that it can support low to medium loads or be used in off-duty conditions.